In an air-conditioner having a refrigerant circuit (refrigeration cycle) represented by a multi air-conditioner for buildings, in which a plurality of load-side indoor units is connected and each indoor unit is operated separately, refrigerator oil is discharged along with a refrigerant from a compressor. In such an air-conditioner, conventionally, an oil separator has been disposed in general at the secondary side (discharge side) of the compressor for the purpose of reducing the distribution amount of the refrigerator oil brought out of the compressor in the refrigeration circuit to immediately return to the compressor. (Refer to Patent Document 1, for example)
Reasons for disposing the oil separator are given as follows. First, as a connecting pipe (refrigerant pipe) that links a heat source unit (outdoor unit) with an indoor unit becomes longer, the amount of refrigerator oil distributed in the connecting pipe increases and a necessary oil amount in the compressor possibly runs short. Second, since a plurality of indoor units separately start/stop, the refrigerator oil is sometimes accumulated in the suspended indoor unit. Third, when the refrigerant stagnates in the compressor and the compressor is started under the condition that oil is diluted, it takes time for the compound liquid of the brought-out refrigerant and refrigerator oil to return to the compressor after circulating in the refrigerant circuit, resulting in the lowering of reliability of the compressor possibly.
In the air-conditioner described in Patent Literature 1, the refrigerator oil brought out of the compressor is adapted to be separated into a high-pressure high-temperature gas refrigerant and refrigerator oil by an oil separator. Then, the high-pressure high-temperature gas refrigerant flows into a heat source side heat exchanger and the separated refrigerator oil is returned to the primary side (suction side) of the compressor under low-pressure low-temperature conditions after being decompressed by a decompression apparatus. At that time, part of the high-pressure high-temperature gas refrigerant is decompressed by the decompression apparatus along with the refrigerator oil and returned to the suction side of the compressor under the low-pressure high-temperature condition at the same time with the refrigerator oil.
Reasons for returning oil to the primary side of the compressor are given as follows. First, the refrigerator oil discharged from the compressor along with the refrigerant and brought out from the compressor needs to be returned to the compressor without delay. Second, the refrigerator oil discharged from the compressor along with the refrigerant and brought out from the compressor needs to be returned to the compressor before the concentration of the refrigerator oil in the compressor becomes extremely lowered.